Nuclear power plants and other facilities handling radioactive material generate radioactively contaminated lubricating oil comprising mixtures of turbine-type, hydraulic or gear oils and lesser amounts of synthetic oils found in general nuclear service. This radioactive waste oil presents a serious disposal problem. To permit off-site disposal, the beta/gamma radioactivity of the waste oil must be reduced to non-detectable levels and the tritium content to below about 2.mu.Ci/kg (740 kBq/kg).
It has been recognized in the case of radioactive waste oil that one approach to reducing the end volume of processed radioactive waste, thereby facilitating its disposal, is to remove the actual radioactive contamination and process it, rather than treat the contaminated oil itself as the radioactive waste needing to be disposed. For example, U.S. Pat. No. 4,615,794 (Belanger) discloses a process in which the waste oil is pre-treated (filtered, heated and skimmed) and an amount of calcium or sodium hypochlorite is added to initiate salt formation with the radioactive contaminants. Treatment with a carbonated pH buffer then converts the nuclide cations into solid salts which can be filtered off.
A number of prior art patents relate to the decontamination of non-radioactive oils by the removal of undesirable (generally toxic) heavy metals such as lead. U.S. Pat. No. 5,286,380 (Mellen) describes a process in which 1 part of contaminated motor oil is mixed with about 10 parts of a suitable solvent such as butane, precipitants are allowed to settle, the solution is percolated through an activated charcoal filter and regenerated oil is separated by vaporizing off the solvent.
The present applicants have discovered a process for greatly reducing the concentration of heavy metals (principally lead and cadmium) from contaminated lubricating oils and especially for reducing radioactivity in such oils to acceptable levels by removal of metal nuclides. The process of the invention is advantageous in employing relatively mild conditions and, unlike prior art methods, requiring no handling of strong oxidants nor the addition of substantial quantities of reagents and/or solvents.
Commercial lubricating oils of the kind used in nuclear service contain, in addition to the base fluid (primarily non-polar, solvent-refined petroleum oil basestock), from about 0.5 to about 5% each of various additives intended to inhibit oxidative breakdown of the oil in use reduce wear, inhibit corrosion and modify rheological properties. Phenolic oxidation inhibitors and zinc-or phosphorous-based antiwear additives are typical. It will be understood throughout this specification that "lubricating oil" refers to such commercial, stabilized products.
From our investigations and experiments, it appears that the gamma-emitting radionuclides present in lubricating oils contaminated during nuclear service are associated not to any significant degree with the base oil itself, but primarily with the aforementioned thermal and oxidation resistance additives. According to the present method, waste lubricating oil heated in the presence of oxygen, a catalyst and a free radical promoter (initiator) forms an "oxidation sludge" containing essentially all of the gamma activity (principally from Co-60), which is believed to arise from the preferential and rapid degradation of the corrosion and oxidation inhibitors present in the lubricating oil.